Convergence calibration in video displays with signal level control

ABSTRACT

A television system performs beam convergence in video displays. The system is implemented through one or more convergence sensors, which are exposed to two distinct convergence test patterns. The output signals from the sensors when exposed to the two convergence test patterns are used in beam convergence calculations. The convergence test patterns are selected to reduce the dynamic range requirement for an A/D converter in the television system supplied with the sensor output signals.

BACKGROUND OF THE INVENTION

The present invention is related to video displays, and moreparticularly to performing convergence calibration in a video displaywith signal level control.

It is well known in the field of video displays to generate pictures ona screen by combining multiple beams of light. For example, a typicalrear projection color television set includes three cathode ray tubes(CRTs), each CRT processing one of the primary colors—red, blue orgreen. By combining the three monochromatic beams the set can producefull color television pictures. However, in order for the set to produceaccurate pictures, proper alignment of the beams must be maintained.That is, the CRTs must be calibrated so that their beams are focused atthe same point on the screen. Accordingly, the calibration of the CRTsis often referred to as a convergence procedure, and beam alignment isoften referred to as convergence. For a more detailed discussion ofconvergence, references are made to FIGS. 1 and 2.

FIG. 1 is a plan view of a model rear projection television set. Thecomponents of the set are housed within a cabinet 10, and they include:a CRT 12, a lens 14, a mirror 16, and a screen 18. The model setincludes three CRTs and multiple lenses for each CRT, although forclarity, only a single CRT and a single lens are shown in the figure.The light from the CRT passes through the lens and illuminates themirror which, in turn, reflects the light onto the screen forobservation by the viewer.

FIG. 2 illustrates the relationship between the three CRTs of the modelset. As can be seen from the figure, CRTs 12, 20 and 22 are matchedrespectively with lenses 14, 24 and 26, and the CRTs are aligned so thattheir beams converge. To maintain the alignment of the beams one or morephotosensors are typically provided at the periphery of the screen. Anexample is shown in FIG. 3.

FIG. 3 includes an arrangement of four photosensors, 28, 30, 32 and 34.The sensors are located inside the cabinet and are not visible to theviewer. Also, the sensors are located behind a screen frame 36, which isnot part of the display screen, and therefore the sensors do notinterfere with images displayed on the screen. Nevertheless, the sensorsare located within the area that can be scanned by the CRTs.

FIG. 4A shows the relationship between sensors 28-34, screen 18, and aCRT scannable area 38 as seen from the viewer's perspective. For claritythe screen frame is not shown. When performing the convergenceprocedure, test patterns are produced within the scannable area anddetected by the sensors. More specifically, each CRT produces two testpatterns, a wide pattern and a narrow pattern. Thus, to complete theconvergence procedure the following patterns are produced: red-wide,red-narrow, blue-wide, blue-narrow, green-wide, and green-narrow. Thesepatterns and their function are discussed in more detail in connectionwith FIGS. 4B-4E.

FIGS. 4B-4E show illustrative test patterns as generated by any one ofthe primary color CRTs. In the interest of brevity, FIGS. 4B-4E arediscussed in the context of the red CRT only. However, it should benoted that the discussion is equally applicable to the other primarycolor CRTs.

FIGS. 4B and 4C show test patterns that are generated when the red CRTis properly aligned with the center of the screen. FIG. 4B shows ared-wide pattern 40 and its relative position to the scannable area,screen, and sensors. As can be seen from the figure, the red-widepattern is made up of four illuminated areas that define a rectangle(indicated by the dotted line). Each illuminated area overlaps theentirety of one sensor. The center point of the scannable area isdenoted by “o” and the center of the rectangle defined by the red-widepattern is denoted by “x”. Since the red CRT is properly aligned, the oand x coincide.

FIG. 4C shows a red-narrow pattern 42. As in the case of the widepattern, since the CRT is properly aligned, the x and o coincide.However, in the case of the narrow pattern, only one half of each of thesensors is overlapped by the pattern. The relative sensor overlap in thewide pattern and narrow pattern cases is key to maintaining alignment ofthe CRT, and will be discussed in more detail below. First, FIGS. 4D and4E are referred to in order to show the effect of misalignment on thetest patterns.

FIG. 4D shows a red-wide pattern 44 that is generated when the red CRTis misaligned by an amount δ in the horizontal direction (left of centerfrom the viewer's perspective). Since the pattern is sufficiently wide,it still overlaps the entirety of each of the sensors. FIG. 4E showsred-narrow pattern 46 that is generated when the red CRT is misalignedby an amount δ in the horizontal direction (left of center from theviewer's perspective). In FIG. 4E, since the pattern is narrow, thesensor overlap is changed relative to the overlap shown in FIG. 4C. Aswill be described below, this change in overlap is used to determine theamount of misalignment, which is, in turn, used as an error signal forthe purpose of correcting the misalignment.

The amount of beam misalignment at a position defined by a given sensoris determined by observing that sensor's readings when exposed to thewide and narrow patterns. The observed readings are used to form a ratiowhich is then compared to a desired ratio, the desired ratio being theratio obtained for the sensor under no misalignment conditions. Thedifference between the measured ratio and the desired ratio indicatesthe amount of beam misalignment. Described below is an illustrativemisalignment determination as performed by sensor 28.

FIGS. 5A-5E show the relationship between sensor 28 and various testpatterns. FIG. 5A depicts the sensor in a no pattern condition. FIGS.5B-5E show the sensor as illuminated by the patterns of FIGS. 4B-4E,respectively. To measure the misalignment, the light incident on sensor28 is measured for each of the wide and narrow cases and a ratio of thetwo is computed. The value of the ratio in the no misalignment case isthe desired ratio, and it is obtained in the following manner: thesensor reading under no pattern conditions (noise) is subtracted fromthe sensor reading under wide-pattern/no-misalignment conditions (FIG.5B) to generate a first difference; the sensor reading under no patternconditions is subtracted from the sensor reading undernarrow-pattern/no-misalignment conditions (FIG. 5C) to generate a seconddifference; and the second difference is divided by the firstdifference. To obtain the value of the ratio for the depictedmisalignment: the sensor reading under no pattern conditions (noise) issubtracted from the sensor reading under wide-pattern/δ-misalignmentconditions (FIG. 5D) to generate a first difference; the sensor readingunder no pattern conditions is subtracted from the sensor reading undernarrow-pattern/δ-misalignment conditions (FIG. 5E) to generate a seconddifference; and the second difference is divided by the firstdifference. The difference between the two ratios thus obtainedindicates the amount of misalignment. The red CRT is then adjusted untilthe ratios match. A similar procedure is executed for the other primarybeams and in this way convergence is achieved.

OBJECTS OF THE INVENTION

It is an object of the present invention to calibrate video displaysaccurately and easily.

It is another object of the present invention to adjust video displayshaving input signal level control.

It is a further object of the present invention to provide convergencecalibration/adjustment with optimum utilization of the dynamic range inA/D converters in video displays.

It is a yet further object of the present invention to provideconvergence calibration/adjustment without supplying either a saturationsignal level or insufficient signal level to A/D converters in videodisplays such that the accuracy of calibration/adjustment is enhanced.

SUMMARY OF THE INVENTION

These and other objects, features and advantages are accomplished byapparatus for performing a convergence calibration operation using anumber of beams. The inventive apparatus includes a signal generator forgenerating first and second video signals for use in the convergencecalibration operation. Further included is a level controller forcontrolling respective levels of the generated first and second videosignals received from the signal generator. The apparatus also comprisesa beam generator for generating first and second beam patterns inresponse to the first and second video signals, respectively, suppliedby the level controller. Also included in the apparatus is at least onesensor for converting the generated first and second beam patterns tofirst and second output signals, respectively, in response to thegenerated first and second beam patterns, respectively. Further, theinventive apparatus comprises a programmable controller for receivingthe first and second output signals and for determining alignment of thebeams based on the received first and second output signals.

In accordance with one aspect of the present invention, the signalgenerator selectively generates a control signal such that the levelcontroller controls respective levels of the generated first and secondvideo signals in response to the control signal received from the signalgenerator. The control signal is active only during the convergencecalibration operation.

In accordance with another aspect of the present invention, the levelcontroller comprises a number of switches and resistors for selectivelyattenuating respective levels of the generated first and second videosignals.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned as well as additional objects, features andadvantages of the present invention will become readily apparent fromthe following detailed description thereof which is to be read inconjunction with the accompanying drawings, in which:

FIG. 1 is a plan view of a typical rear projection television set;

FIG. 2 illustrates the relationship between the three CRTs of the FIG. 1set;

FIG. 3 shows how photosensors are typically arranged around a screen forpurposes of maintaining beam convergence;

FIG. 4A shows the typical relationship between a plurality ofconvergence sensors, a display screen, and a CRT scannable area;

FIGS. 4B-4E show the typical relationship between the sensors, thedisplay screen, the scannable area, and several test patterns;

FIG. 5A is a representation of an unilluminated sensor;

FIGS. 5B-5E are representations of the sensor of FIG. 5A as illuminatedby the test patterns of FIGS. 4B-4E, respectively;

FIGS. 6A-6D show the relationship between the sensors, the displayscreen, the scannable area, and several test patterns in accordance withthe present invention;

FIG. 7A is a representation of an unilluminated sensor;

FIGS. 7B-7E are representations of the sensor of FIG. 7A as illuminatedby the test patterns of FIGS. 6A-6D, respectively;

FIG. 8 is a schematic diagram of a television system according to thepresent invention;

FIGS. 9A and 9B are graphs of output signals from the R, G, B sensorsversus time in response to constant levels of three beams of light asemitted by the R, G, B CRTs, respectively;

FIG. 10 is a graph of decay characteristics of phosphor responsive toprimary R, G, B colors versus time;

FIG. 11 is a graph of sensor sensitivity versus light beam wavelength asreceived by a sensor;

FIG. 12 shows a block diagram of the television screen displaying apicture (a so-called non-test pattern) simultaneously with theconvergence test pattern directed at the sensors located just outsidethe left and right edges of the television screen;

FIGS. 13A and 13B are graphs of signal levels provided by the videoprocessor during test and non-test patterns;

FIGS. 13C and 13D are graphs of signals output by the sensors incorrespondence with the signals provided by the video processor asillustrated in FIGS. 13A and 13B;

FIG. 14 shows a representative embodiment of a level controller inaccordance with one aspect of the present invention;

FIGS. 15A-E are graphs of various input and output signals provided bythe pattern generator 101, level controller 106, video processor 100 andsensors 64, 66, 68, 70 in accordance with the present invention; and

FIG. 16 shows the relationship between sensors, display screen, andscannable area for an alternative sensor arrangement in accordance withthe present invention.

In all Figures, like reference numerals represent the same or identicalcomponents of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 6A-6D show illustrative beam convergence test patterns generatedaccording to a preferred embodiment of the invention. Although the testpatterns depicted in the figures may be generated by any one of theCRTs, they will be discussed in the context of the red CRT for clarityof presentation.

FIGS. 6A and 6B show the test patterns generated by the red CRT when itis properly aligned. FIG. 6A shows a first test pattern 48 whichoverlaps a left side portion (from the viewer's perspective) of eachsensor. FIG. 6B shows a second test pattern 50 which overlaps a rightside portion of each sensor. As was the case in FIGS. 4A-4E, “o”indicates the center of the screen and “x” indicates the center of therectangle defined by the test patterns. The “o” and “x” coincide inFIGS. 6A and 6B since these figures represent the case of proper beamalignment.

FIGS. 6C and 6D show a shifted first test pattern 52 and a shiftedsecond test pattern 54, respectively. The shifted first pattern is thepattern that results when the CRT is misaligned and attempts to generatethe first pattern. The shifted second pattern is the pattern thatresults when the CRT is misaligned and attempts to generate the secondpattern. In both of FIGS. 6C and 6D, the CRT is misaligned by an amountδ in the horizontal direction (left of center from the viewer'sperspective).

As can be seen from FIGS. 6A-6D, the effect of the misalignment on thefirst pattern is to shift the pattern (FIG. 6C) so that its overlap withsensor 28 significantly less than it was in the no-misalignment case(FIG. 6A). Whereas the effect of the misalignment on the second patternis to shift the pattern (FIG. 6D) so that its overlap with sensor 28 issignificantly greater than it was in the no-misalignment case (FIG. 6B).As will be described below, the overlap changes for the two testpatterns that occur as a result of misalignment are used to determinethe amount of misalignment. The amount of misalignment is then, in turn,used as an error signal for the purpose of correcting the misalignment.

The amount of misalignment is determined by observing the sensorreadings as the sensor is exposed to the first and second patterns. Thereadings obtained are used to form a measured ratio that is compared toa desired ratio, the desired ratio being the ratio obtained for thesensor under no-misalignment conditions. The difference between themeasured ratio and the desired ratio indicates the amount of beammisalignment at the sensor's location. What follows is a description ofan illustrative misalignment determination as performed on the basis ofreadings taken through sensor 28.

FIGS. 7A-7E show the relationship between sensor 28 and various testpatterns. FIG. 7A depicts the sensor in a no pattern condition. FIGS.7B-7E show the sensor as illuminated by the patterns of FIGS. 6A-6D,respectively. To measure the misalignment, the light incident on sensor28 is measured for each of the first pattern and second patternmeasurements to form a ratio. The value of the ratio in theno-misalignment case is the desired ratio, and it is a design parameterfor the television set. To obtain the value of the desired ratio: thesensor reading under no pattern conditions (noise) is subtracted fromthe sensor reading under first-pattern/no-misalignment conditions (FIG.7B) to generate a first difference; the sensor reading under no patternconditions is subtracted from the sensor reading undersecond-pattern/no-misalignment conditions (FIG. 7C) to generate a seconddifference; the first difference is added to the second difference toform a sum; and the first difference is divided by the sum. To obtainthe value of the ratio for the depicted misalignment: the sensor readingunder no pattern conditions (noise) is subtracted from the sensorreading under first-pattern/δ-misalignment conditions (FIG. 7D) togenerate a first difference; the sensor reading under no patternconditions is subtracted from the sensor reading undersecond-pattern/δ-misalignment conditions (FIG. 7E) to generate a seconddifference; the first difference is added to the second difference toform a sum; and the first difference is divided by the sum. Thedifference between the two ratios thus obtained indicates the amount ofmisalignment. The red CRT is then adjusted until the ratios match. Asimilar procedure is executed for the other primary beams and in thismanner convergence about sensor 28 is achieved. Finally, similarprocedures may then be executed about the other sensors to complete anoverall convergence procedure.

By using the test patterns of the present invention the dynamic rangerequired of the sensor A/D converters is reduced, thereby allowingconvergence procedures to be performed through more cost efficienthardware. To illustrate how the dynamic range requirement is reducedreference is made to FIGS. 5A-5E and 7A-7E.

As mentioned above, A/D converters are employed to convert the output ofconvergence sensors from analog signals to digital signals. In priorsystems, the analog sensor signals that had to be converted by the A/Dconverters ranged from the signal generated when the sensor was fullyilluminated to the signal generated when the sensor was not illuminatedat all. For example, in prior systems, the signal obtained from theunilluminated sensor in FIG. 5A and the signal from the fullyilluminated sensor in FIG. 5D are both converted to digital signals sothat they can be used to calculate the convergence ratio. However, inthe present invention, a sensor A/D converter does not have to convertthe signal from a fully illuminated sensor.

In a preferred embodiment of the present invention, the first and secondtest patterns are chosen such that for any given sensor the areas of thesensor overlapped by the first and second patterns are complimentary.That is, the area not overlapped by the first pattern is equal to thearea overlapped by the second pattern; and the area not overlapped bythe second pattern is equal to the area overlapped by the first pattern.Thus, a value for a fully illuminated sensor may be obtained by addingthe digitally converted output generated during illumination by thefirst pattern to the digitally converted output generated duringillumination by the second pattern, without actually fully illuminatingthe sensor. Indeed, as will be explained with references to FIGS. 7B-7E,the maximum sensor illumination is close to 50%.

Regarding sensor illumination in the no-misalignment case, it can beseen from FIGS. 7B and 7C that the maximum signal that must be convertedin the no-misalignment case is the signal corresponding to approximately50% illumination. Regarding sensor illumination in the δ-misalignmentcase, it can be seen from FIG. 7E that the maximum signal that must beconverted is somewhat more than the signal corresponding toapproximately 50% illumination. However, the amount of misalignment δ istypically small compared to the size of the sensor, and therefore themagnitude of the signal will not be significantly larger than the signalcorresponding to 50% sensor illumination. Thus, the reduction in A/Ddynamic range requirement provided by the present invention is on theorder of 50%.

A schematic diagram of a television system according to the invention isshown in FIG. 8. The system includes a television screen 60 and threeCRTs 82, 84 and 86 for forming images on the screen. Each CRT emits abeam of monochromatic light in one of the primary colors (red, blue andgreen), and each includes a deflection yoke, 88, 90 and 92,respectively. Control of the CRTs 82, 84, 86 for the purpose of formingimages on the screen is performed by a video processor 100. Connected tothe video processor 100 is a pattern generator 101 via a levelcontroller 106. The pattern generator 101 generates R, G, B signals thatdefine either convergence test patterns during the internal convergenceadjustment/calibration procedure or non-test patterns (pictures) fordisplay on the screen 60. The level controller 106 controls the levelsof the signals which are output by the pattern generator 101, asexplained in detail hereinbelow. Convergence adjustment of the CRTs 82,84, 86 is performed through a deflection yoke driver 80. Since theinvention concerns convergence operations, the convergence portion ofthe system will be emphasized.

The system includes four convergence photosensors 64, 66, 68 and 70.These sensors are located at the periphery of the screen, behind ascreen frame 62. During convergence operations, the sensors 64, 66, 68,70 generate analog current signals which are passed to acurrent-to-voltage (I/V) converter 72. The current-to-voltage converter72 converts the current signals to analog voltage signals and relays thevoltage signals to an A/D converter 74. The A/D converter 74 receivesthe analog voltage signals, converts them to digital voltage signals,and passes the digital voltage signals to a programmable controller 76.The controller 76 then uses the digital voltage signals to perform theconvergence calculations and determine the amount of any necessary beamalignment corrections. If correction is required, the controller 76sends appropriate correction signals to a diffraction wave generator 78.The correction signals received by the diffraction wave generator 78 areconverted into driver signals which are, in turn, passed to thedeflection yoke driver 80. The deflection yoke driver 80 then generatesone or more deflection yoke control signals and applies them to the CRTdeflection yokes 88, 90 and 92. Through repeated beam adjustment by wayof the deflection yokes 88, 90 and 92, proper beam alignment ismaintained.

In addition to the above-identified elements, the television system ofFIG. 8 includes an antenna 96, a tuner 94 and an infrared receiver 102.The tuner 94 receives television signals, such as broadcast signals,through the antenna 96 that are coupled to the video processor 100.Infrared receiver 102 is provided to allow for remote control of thesystem via remote control unit 104.

In a television system using the inventive calibration/adjustmentcircuits as described hereinabove and shown in FIG. 8, it is importantto set the analog signal (voltage) at a proper level supplied to the A/Dconverter 74. That is, if the input signal level from thecurrent-to-voltage converter 72 is too high, the A/D converter 74 willbecome saturated and, among other things, the operation of thecalibration/adjustment procedure will be erroneous because the positionsof the beams will not be calculated accurately. On the other hand, ifthe input signal level from the current-to-voltage converter 72 is toolow, the conversion by the A/D converter 74 also will be imprecise.Namely, because of the low input signal level which may be additionallyinfluenced by a noise signal level, the resolution of the A/D converter74 will be so low that the conversion accuracy will be significantlyaffected. For this reason, the signal output by the current-to-voltageconverter 72 should be kept at an optimum level, and preferably justbelow the maximum (saturation) level of the A/D converter 74.

It has been recognized that output signal levels of red, green and bluesensors, such as the convergence photosensors 64, 66, 68 and 70, varyeven if an input signal level is constant or substantially the same. Asshown in FIG. 9A, when the input signal to the photosensors ismaintained at substantially the same level to generate the red, greenand blue light beams by the CRTs 82, 84 and 86, respectively, the R, G,B sensors (such as the convergence photosensors 64, 66, 68 and 70)produce different output signal levels based on primary colors, as shownin FIG. 9B. Namely, FIG. 9B shows that the highest signal output isproduced by the photosensors responding to a blue light beam, followedby the photosensors responding to a green light beam and finally a redlight beam. This difference in output signal levels is due to variationsin decay characteristics of each phosphor corresponding to the R, G, Bprimary colors as shown in FIG. 10. Further, FIG. 11 illustrates arelationship between sensitivity of a photosensor and a light beamhaving various wavelengths to which the photosensor responds, therebyindicating another reason for the variations in the output signal levelsof the sensors.

To compensate for the variations in output signal levels of the R, G, Bsensors as indicated hereinabove, it has been suggested to use the videoprocessor 100. That is, under control of program instructions, eitherfrom external or internal memory, the video processor 100 adjusts the R,G, B signal levels accordingly.

Several disadvantages, however, accompany the above-mentioned type ofsignal level control. For instance, different types of video processorsmay be used in video systems requiring specific programs (software) tobe individually created for each type of video processor. In addition tosacrificing uniformity in software, a long response time negativelyaffects processing speed in the video system as the video processorexecutes time-intensive software routines for each of the R, G, Bsensors.

A further disadvantage is that the above-mentioned software controlledlevel adjustment may change the entire display on a screen. As shown inFIG. 12, a picture (a so-called non-test pattern) as representativelyshown by the display “AUTO” may be displayed on the television screen 60at the same time as the convergence test pattern is directed at thesensors, as representatively shown being located just outside the leftand right edges of the television screen 60. In this situation,brightness of the display “AUTO” is undesirably changed in accordancewith specific program instructions directed to a particular color in thetest pattern. Since the video processor does not distinguish between thetest and non-test patterns and controls the signal level based only onthe R, G, or B color, the picture display is affected.

FIGS. 13A and 13B show the input signal level maintained by the videoprocessor 100 at substantially the same level for test and non-testpatterns alike (representatively shown for red and blue light beams asgenerated by the CRTs 82 and 86, respectively). The corresponding redand blue sensors, such as the convergence photosensors 64, 66, 68 and70, produce the same output signal levels for test and non-testpatterns, as shown in FIGS. 13C and 13D. Thus, the software-controlledsignal level adjustment by the video processor 100 is disadvantageousfor the reasons as stated hereinabove.

To overcome the above disadvantages associated with thesoftware-controlled signal level adjustment due to the variations inoutput signal level of the sensors, in accordance with one aspect of thepresent invention, the level controller 106 is provided in thetelevision system of FIG. 8. During the calibration procedure, the levelcontroller 106 receives R, G, B signals for forming test or non-test(picture) patterns and a control signal from the pattern generator 101,controls the respective level of the received R, G, B signalsaccordingly, and outputs them to the video processor 100 for furtherprocessing as previously explained.

One representative embodiment of the level controller 106 is shown inFIG. 14. The level controller 106 can be built inexpensively by usingswitches and resistors having appropriate values to obtain apredetermined signal level corresponding to each primary color R, G, Bsuch that A/D converters (such as the A/D converter 74) are providedwith optimum signal level, that is neither saturated nor supplied with avery low input level. As further shown in FIG. 14, the switches arecontrolled by the control signal supplied from the pattern generator 101(FIG. 8). In response to the control signal, the switches may be in theclosed (on) position or open (off) position.

In operation, during the convergence calibration/adjustment operationsas described hereinabove in accordance with the present invention, R, G,B signals defining a preselected test pattern for calibration aregenerated by the pattern generator 101 as shown in FIG. 8, and the R, G,B signals are supplied to the level controller 106. In addition, thepattern generator 101 activates the control signal supplied to the levelcontroller 106. In response to the control signal, the level controller106 adjusts the R, G, B signals to an optimum level. In particular, theswitches in the level controller 106 are closed (turned ON) by thecontrol signal, whereby the respective levels of the R, G, B signals arechanged (attenuated) based on a predetermined resistor selection, forexample, as representatively shown in FIG. 14. Following the processingby the level controller 106, the R, G, B signals are output to the videoprocessor 100.

When the convergence calibration/adjustment procedure is not performed,that is, during the picture display on the screen 60, the patterngenerator 101 de-activates the control signal supplied to the levelcontroller 106. As a result, the level controller 106 merely passes theR, G, B signals through without modification. The switches in the levelcontroller 106 become open (turned OFF), whereby the respective level ofeach R, G, B signal is passed through to the video processor 100 asshown in FIG. 14.

FIGS. 15A-E are graphs of various input and output signals provided bythe pattern generator 101, level controller 106, video processor 100 andsensors 64, 66, 68, 70 in accordance with the present invention asdescribed hereinabove. As representatively shown in FIG. 15A, thepattern generator 101 generates signals having predetermined levels anddefining B, G test patterns and a non-test (picture) pattern. FIG. 15Bshows the control signal as supplied to the level controller 106 by thepattern generator 101. The control signal is activated only during theconvergence calibration operation, as representatively shown for theblue and green test patterns. FIG. 15C shows the input signals to thevideo processor 100 following the processing by the level controller106. The levels of B and G signals defining the corresponding testpattern are attenuated to attain proper level, while the signal definingthe non-test pattern (picture) remains at the same level as prior to theprocessing by the level controller 106. FIGS. 15D and 15E are graphs ofsignals produced by the representative left and right sensorscorresponding to the B and G signals as supplied by the video processor100 to the respective CRTs. The signals output by the sensors are atsuch levels as to obtain maximum resolution in the A/D converters in thevideo system.

In accordance with the present invention, the signal levels arecontrolled in a simple, efficient and cost effective manner. Neitheradditional and time-consuming software for the video processor norcomplicated hardware is required to attain proper level control of thevideo signals.

It is understood that other embodiments will be readily apparent tothose skilled in the art based on the teaching herein. For example, forone or more sensors used in a convergence procedure, the area overlappedby the first test pattern when added to the area overlapped by thesecond test pattern does not need to be equal to the entirephotosensitive area (surface) of the sensor(s), but rather may be equalto some other predetermined area.

Moreover, sensor arrangements other than those described above may beemployed. For example, sensors may be arranged as shown in FIG. 16. Inthe FIG. 16 configuration, five sensors are employed, 402-410, sensor410 being located at the center of the screen. Convergence testing isconducted in the same manner as described above with the exception thatthe test patterns include five illuminated areas.

Furthermore, the invention is applicable to many types of video displaysand is not limited to rear projection television sets. For example, theinvention may be employed in computer system monitors.

Having described specific preferred embodiments of the invention withreference to the accompanying drawings, it is to be understood that theinvention is not limited to those precise embodiments, and that variouschanges and modifications may be effected therein by one skilled in theart without departing from the scope or the spirit of the invention asdefined in the appended claims.

What is claimed is:
 1. A system for performing a convergence calibrationoperation using at least one beam, comprising: a level controller forselectively controlling a level of an input video signal only when saidinput video signal includes a test pattern signal; a beam generator forgenerating a beam pattern in response to said input video signalsupplied by said level controller; at least one sensor for convertingthe generated beam pattern to an output signal; and a programmablecontroller for receiving said output signal and for determiningalignment of said at least one beam based on the received output signal.2. A method for performing a convergence calibration operation using atleast one beam, comprising the steps of: selectively controlling a levelof an input video signal only when said input video signal includes atest pattern signal; generating a beam pattern in response to said inputvideo signal; converting the generated beam pattern to an output signal;receiving said output signal; and determining alignment of said at leastone beam based on the received output signal.
 3. Apparatus forperforming a convergence calibration operation using a number of beams,comprising: a signal generator for generating first and second videosignals for use in said convergence calibration operation; a levelcontroller for selectively controlling respective levels of thegenerated first and second video signals received from said signalgenerator only when said respective first and second video signalsinclude a test pattern signal; a beam generator for generating first andsecond beam patterns in response to said first and second video signals,respectively, supplied by said level controller; at least one sensor forconverting the generated first and second beam patterns to first andsecond output signals, respectively, in response to the generated firstand second beam patterns, respectively; and a programmable controllerfor receiving said first and second output signals and for determiningalignment of the beams based on the received first and second outputsignals.
 4. Apparatus for performing a convergence calibration operationusing a number of beams, comprising: a signal generator for generatingfirst and second video signals for use in said convergence calibrationoperation; a level controller for controlling respective levels of thegenerated first and second video signals received from said signalgenerator; a beam generator for generating first and second beampatterns in response to said first and second video signals,respectively, supplied by said level controller; at least one sensor forconverting the generated first and second beam patterns to first andsecond output signals, respectively, in response to the generated firstand second beam patterns, respectively; and a programmable controllerfor receiving said first and second output signals and for determiningalignment of the beams based on the received first and second outputsignals, wherein said signal generator selectively generates a controlsignal such that said level controller controls said respective levelsof the generated first and second video signals in response to saidcontrol signal received from said signal generator.
 5. The apparatusaccording to claim 4, wherein said control signal is active only duringsaid convergence calibration operation.
 6. The apparatus according toclaim 4, wherein said level controller comprises a number of switchesand resistors for selectively attenuating said respective levels of thegenerated first and second video signals.
 7. The apparatus according toclaim 6, wherein said signal generator generates said control signal toclose the switches only during said convergence calibration operation.8. The apparatus according to claim 3, wherein said at least one sensorhas at least first and second portions of a sensitive surface exposed tothe beams such that said first and second beam patterns generated bysaid beam generator overlap said first and second portions,respectively.
 9. The apparatus according to claim 8, wherein said firstand second portions comprise the entire sensitive surface of said atleast one sensor.